Output transistor protecting system in a transistor amplifier circuit



y 13, 1967 FUJIO SUGANUMA 3,332,027

OUTPUT TRANSISTOR PROTECTING SYSTEM IN A TRANSISTOR AMPLIFIER CIRCUIT Filed Aug. 20, 1963 2 Sheets-Sheet 1 TR RSI 1" F79.

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OUTPUT TRANSISTOR PROTECTING SYSTEM IN A TRANSISTOR AMPLIFIER CIRCUIT Filed Aug. 20, 1963 2 Sheets-Sheet 2 INPUT Slg INPUT Sig INVENTOR FUJIO SUGANUMA United States Patent 3,332,027 OUTPUT TRANSISTOR PROTECTING SYSTEM IN A TRANSISTOR AMPLIFIER CIRCUIT Fujio Suganuma, Tokyo, Japan, assignor to TDK Electronics Company, Limited, and Toyo Music Broadcasting Company, Limited, both of Tokyo, Japan, both corporations of Japan Filed Aug. 20, 1963, Ser. No. 303,302 Claims priority, applicationllapan, Aug. 28, 1962, 37/ 36,776 6 Claims. (Cl. 33013) This invention relates to an output transistor protecting system for preventing transistors from being ruined by an excess input, electric pulse input or load shortcircuiting in a transistor amplifier output circuit.

Various circuits have been already suggested for such kind of output transistor protecting system. However, if a resistor or the like is inserted in the output circuit to protect the transistor, the transistor will be protected but with an undesirable reduction in the characteristics of the circuit of the transistor. The present invention is suggested to eliminate the above mentioned defect.

A principal object of the present invention is to provide an output transistor protecting system wherein an operating current flowing to both the amplifying transistor and the output transistor can be so restricted that the flow is not above the value required for the maximum normal outputs of said transistors and the transistors can be perfectly prevented from being damaged by an excess input, electric pulse input or load short-circuiting without influencing the normal amplifying action and other characteristics.

FIGURE 1 shows an embodiment of the present invention.

FIGURES 2, 3, 5, 6 and 7 show other embodiments of the present invention.

FIGURE 4 is an explanatory diagram for setting the value of a protective resistance to be used in the system of the present invention.

An embodiment of the present invention shall now be explained with reference to the drawings. FIGURES 1, 2, 3, 5, 6 and 7 show protective resistor of the present invention as applied to output circuits for transistorized audio frequency amplifiers.

In FIGURE 1, a protective resistor RS is provided 7 between the respective collectors of a PNP type front stage transistor TR and a PNP type rear stage transistor TR That is to say, an input signal is applied to the base and emitter of the front stage transistor TR through the rear stage transistor TR and a resistor R'B The emitter of the front stage transistor TR and the base of the rear stage transistor TR are connected with each other. The resistor RB is connected between the emitter and base of the rear transistor TR A protective resistor RS is inserted between the respective collectors of the transistors TR, and TR The minus side of a battery is connected to the collector of the transistor TR The plus side of the battery is connected to the emitter of the transistor TR through load Z.

In the circuit shown in FIGURE 2, a protective resistor RS is inserted and provided between the collector of an NPN type front stage transistor TR, and the connecting point P of a shunt resistor R'B connected to the base of a PNP type rear stage transistor TR That is to say, the emitter of the transistor TR the collector of the transistor TR; and a load Z are connected with .one another, an input signal is applied between the base and emitter of the transistor TR and a protective resistor RS is inserted between the connecting point P of the base of the transistor TR; and the shunt resistor RB; and the collector of the transistor TR A load and a battery are connected in series between the emitter and 3,332,927 Patented July 18, 1967 ICC collector of the transistor TR The plus side of the battery is connected to the emitter of the transistor TR, and the minus side of the battery is connected to the collector of the transistor TR through the load.

FIGURE 3 shows a single ended push-pull circuit made by combining the circuits shown in FIGURES 1 and 2. A protective resistor RS is inserted and provided between the respective collectors of a PNP type front stage transistor TR and a PNP type rear stage transistor TR A protective resistor R8,; is inserted between a front stage NPN type transistor TR, and the connecting point P of a shunt resistor RB connected to the base of a PNP type rear stage transistor TR The present invention is based on the following idea. The damage to a transistor is due to an excess current. In order to prevent damage to the transistor TR any excess current above the normal value may be prevented from flowing to the transistor TR The same can be said of the transistor TR In the Darlington circuit in FIGURE 1, if Ib is a base current of the transistor TR Ie is an emitter cur- IC =I81Ib1, I1=Ib2+I But the currents Ib and I can be considered to be of a very small value in practical uses. Therefore,

Therefore, in the output transistor TR in order to prevent the collector current 10 from exceeding the maximum collector current Ic max. required for the maximum normal output the base current 112 may be controlled to not exceed the maximum base current Ib max. of the transistor TR required therefor. In order to restrict the base current Ib the emitter current Ie or the collector current [0 may be restricted by the Formula 1. Therefore, in FIGURE 1, if the current 10 or I2 is restricted so as not to exceed the maximum current Ib max. required by the transistor TR for the maximum normal output, the transistors TR and TR; will be protected from being ruined by an excess input, electric pulse input of load short-circuiting.

In the circuit in FIG. I, when the protective resistor RS is not inserted the load characteristic of the transistor TR is represented by such load line as is shown in 'FIG. 4(B), but the load characteristic can be made to be as represented by the load line in FIG. 4(A) by inserting the protective resistor RS into said circuit.

Even if the collector cur-rent Ic of the transistor TR; is to increase excessively, due to the voltage drop by the protective resistor RS the collector voltage E0 of the transistor TR will drop and will become substantially zero at the maximum collector current required for the transistor TR therefore the collector current I0 will no more increase and the transistor TR, will be protected from being damaged by the excess input and pulse input. At the same time, as described above, as Ic Ibthe base current Ib of the transistor TR will not increase to be more than the specified value Ib max.

( Z Ic max), therefore the collector current Ic will also i the collector current Ic will also increase. However, as described above, said collector current I will be restricted by the protective resistor RS and will not exceed the normal value. Therefore, even if the load is shortcircuited, the collector current also will not exceed the normal value. Thus both transistors TR and 'TR will be protected from being damaged.

As regards the influence of the protective resistor RS in the normal operating condition, it is likely to be thought, when the resistor RS is inserted in the collector of the transistor TR the voltage fluctuation of the transistor TR will become so large as to give a bad influence on the normal operation. However, if the operation of the transistors TR, and TR is considered in the light of the new idea of the present invention based on the concept that the operation of the transistor depends entirely on the current only and that, in the Darlington circuit, though the transistor acts singly, it can be fed with currents from two current sources. It will be found that, as the resistor RS is inserted, when an input is applied to the transistor T R the voltage between the collector and emitter of the transistor TR will naturally be reduced. But as seen in the load characteristic in FIG. 4(A) even when the voltage between the collector and emitter of the transistor T R, is the lowest, the current is maximum. Further, in the transistor TR as described above, the base current Ib (=Ic will not be influenced by the protective resistor RS the collector current 10 can be fed from another current source than of the transistor TR and therefore the collector current I0 will be independent of the protective resistor RS and will have no influence on the output of the transistor TR As a result, only when the normal value is exceeded, the protective resistor RS will operate and will act as a limiter. But it will have no influence at all on the normal operation. Said protective resistor RS is inserted and provided on the collector side of the transistor TR because the operating condition of the original circuit will not be varied by the insertion of the protective resistor RS there. If the protective resistor RS is inserted and provided on the emitter side of the transistor TR the input impedance of the transistor TR will become higher than before the insertion and the operating condition will be different from that of the original circuit. Further, if the protective resistor is inserted and provided on the base side of the transistor TR the impedance connected between the base and emitter of the transistor TR will rise, therefore the voltage (V will reduce between the collector and emitter, the same not being desirable. However, when the protective resistor RS is inserted and provided on the collector side, all above mentioned defects will be eliminated. However, in case the input impedance is desired from the first to be higher than in the circuit of the embodiment in FIGURE 1, the protective resistor RS may be inserted and provided on the emitter side of the transistor TR A method of setting the resistance value of the protective resistor RS of the present invention shall be explained by means of FIGURE 4. In the diagram of Ec-Ic characteristics in which the abscissa represents the collector voltage E0 of the transistor TR and the ordinate represents the collector current 10 of the transistor TR now if the value of the base current Ib required for the output transistor TR to develop the maximum normal output is 10 ma., as the emitter of the amplifying transistor TR and the base of the output transistor TR; are directly connected as mentioned before, the maximum collector current Ic of the amplifying transistor TR required for the output transistor TR to develop the maximum normal output will be, from the Equation 1, 10 ma. Further, if the load line according to the load resistance RL (a parallel value of the resistor RB and input resistance of the output transistor TR and the collector voltage Ecc fed into the amplifying transistor TR will be as shown by B in FIG. 4, the current 16 on the load line will be enough to be within the range not exceeding 10 ma. which the value will be enough to obtain the maximum output, and any current more than that will not be required. It is, therefore, necessary only to make the collector voltage zero when the current I0 flowing through the amplifying transistor TR is 10 ma. so that no current more than that will flow. In other words, it is only necessary to obtain an operation characteristic as shown by A in FIG. 4 of the load line.

Therefore, the load line value RL then will be RL'=Ecc /Ir:,,,,,, (4) and the value of the protective resistor RS will be RS =RL'RL (5) This is the resistance value of the protective resistance The present invention has the above mentioned operating principle. As shown in the embodiment in FIGURE 2, in the Darlington circuit in which the front input transistor is the NPN type transistor TR when the protective resistor RS is inserted and provided between the collector of the transistor TR and the connecting point P of the shunt resistor Rb connected to the base of the PNP type transistor TR the same as in the above described case that the resistor RS is inserted and provided on the collector side of the transistor TR in FIGURE 1, the protective resistor will have no influence on the normal operating condition and will perform a protective action to prevent damage. This is because, as the protective resistor R8,, is inserted and provided on the collector side of the transistor TR the input impedance will not vary and, as it is inserted and provided between the connecting point P of the shunt resistor RB- and the base of the transistor TR, and the collector of the transistor TR the transistor T R, also will not be influenced by the protective resistor RS Further, in such push-pull circuit, too, as is shown in the embodiment in FIGURE 3, as it is a circuit made by combining the circuits shown in FIGS. 1 and 2, when the protective resistors RS and R8,; are inserted in the manner corresponding to the showing in FIGS. 1 and 2, respectively, they will operate the same as is mentioned above and will protect the transistors TR TR TR7 and TR,;.

Further, in case it is necessary to make the input impedance higher on the input terminal side, it is suggested to make the following alterations in the circuits of FIGS. 13 according to the present invention that:

In the circuit as shown in FIG. 1 before described, it is enough to insert, as shown in FIG. 5 the protective resistor RS between emitter of the amplifying transistor TR and the junction of the base of the output transistor TR and the shunt resistor RB In the circuit as shown in FIG. 2, it is enough to insert, as shown in FIG. 6, the protective resistor RS between the emitter of the amplifying transistor TR and the collector of the output transistor TR In the circuit as shown in FIG. 3, it is enough to insert, as shown in FIG. 7, the protective resistor RS between the emitter of the PNP type amplifying transistor TR and the junction of the base of the output transistors TR to be connected with said transistor TR and the shunt resistor RB and to insert the protective resistor R8,, between the emitter of the NPN type amplifying transistor TR and the collector of the output transistor TR to be connected with said transistor TR The input impedance will be able to be made higher in the circuits as shown in FIGS. 5-7 and yet the protecting action and other operations will be the same as in the before mentioned embodiments shown in FIGS.

As the present invention has such operation and formation as are explained above, transistors can be protected and prevented from being damage by an excess input, electric pulse input or load short-circuiting without any influence at all on the normal operation. When it is combined with a conventional temperature compensating circuit system or the like, the operation of the output transistor of the transistor amplifier will be perfectly protected. The range of its utilization is so wide that the present invention is really effective.

What is claimed is:

1. A transistor output stage protecting circuit system in a so-called PNP type Darlington-connected circuit of a fundamental formation in which a load and an electric source are connected in series between the collector and emitter of a PNP type output transistor TR the collector of a PNP type amplifying transistor TR; is connected to the collector of said PNP type output transistor TR the emitter of said PNP type amplifying transistor TR is connected to the base of said PNP type output transistor TR a shunt resistor RB is connected between the base and emitter of said PNP type output transistor TR and between the base of said PNP type amplifying transistor TR and the emitter of said PNP type output transistor TR is made an input terminal, the improvement which comprises inserting between the collector of said PNP type amplifying transistor TR and the collector of said PNP type output transistor TR in said circuit a protective resistor RS of such value that the voltage between the emitter and collector of said PNP type amplifying transistor TR may be substantially zero when the maximum allowable current flows to said PNP type output transistor TR so that said PNP type output transistor TR may be prevented from being damaged by any excess current in its abnormal operating state without directly limiting its output current.

2. A transistor output stage protecting circuit system in a so-called NPN type Darlington-connected circuit of a fundamental formation in which a load and an electric source are connected in series between the collector and emitter of a PNP type output transistor TR the emitter of an NPN type amplifying transistor TR is connected to the collector of said PNP type output transistor TR the collector of said NPN type amplifying transistor TR is connected to the base of said PNP type output transistor TR a shunt resistor RB is connected between the base and emitter of said PNP type output transistor TR; and between the base and emitter of said NPN type amplifying transistor TR is made an input terminal, the improvement which comprises inserting between the collector of said NPN type amplifying transistor TR and the junction of the base of said PNP type output transistor TR; and the shunt resistor RB in said circuit a protective resistor RS of such value that the voltage between the emitter and collector of said NPN type amplifying transistor TR may be substantially zero when the maximum allowable current flows to said PNP type output transistor TR so that said PNP type output transistor TR may be prevented from being damaged by any excess current in its abnormal operating state without directly limiting its output current.

3. A transistor output stage protecting circuit system in a so-called Darlington-connected complementary type output transformerless single-ended push-pull circuit of a fundamental formation in which an electric source is connected between the collector of one TR of two PNP type output transistors TR'] and TR and the emitter of the other transistor TR the collector of said PNP type output transistor TR to whose emitter is connected the electric source is connected to the emitter of said PNP type output transistor TR; to whose collector is connected the electric source, a load is connected through a condenser between the emitter of said PNP type transistor TR7 and the emitter of said PNP type transistor TR the collector of a PNP type amplifying transist-ofTR is connected to the collector of said PNP type output transistor TR the base of said PNP type output transistor TR, is connected to the emitter of said PNP type amplifying transistor TR a shunt resistor R3 is connected between the base and emitter of said PNP type output transistor TR the emitter of an NPN type amplifying transistor TR is connected to the collector of said PNP type output transistor TR the base of said PNP type output transistor TR is connected to the collector of said NPN type amplifying transistor TR a shunt resistor RB is iconnected between the base and emitter of said PNP type output transistor TRg, the base of said NPN type amplifyiug transistor TR is connected to the base of said PNP type amplifying transistor TR and between the base of said PNP type amplifying transistor TR and the emitter of said PNP type output transistor TR7 is made an input terminal, the improvement which comprises inserting between the collector of said PNP type amplifying transistor TR and the collector of said PNP type output transistor TR- in said circuit a protective resistor RS of such value that the voltage between the emitter and collector of said PNP type amplifying transistor TR may be substantially zero when the maximum allowable current flows to said PNP type output transistor TR7 and between the collector of said NPN type amplifying transistor TR and the junction of the base of said PNP type output transistor TR and the shunt resistor RB a protective resistor RS of such value that the voltage between the emitter and collector of said NPN type amplifying transistor TR may be substantially zero when the maximum allowable current flows to said PNP type output transistor TR so that said two PNP type output transistors TR7 and TR may be prevented from being damaged by any excess current in their abnormal operating state without directly limiting their output current.

4. A transistor output stage protecting circuit system in a so-called PNP type Darlington-connected circuit of a fundamental formation in which a load and an electric source are connected in series between the collector and emitter of a PNP type output transistor TR the collector of a PNP type amplifying transistor TR is connected to the collector of said PNP type output transistor TR the emitter of said PNP type amplifying transistor TR is connected to the base of said PNP type output transistor TR a shunt resistor RB is connect-ed between the base and emitter of said PNP type output transistor TR and between the base of said PNP type amplifying transistor TR and the emitter of said PNP type output transistor TR is made an input terminal, the improvement which comprises inserting between the emitter of said PNP type amplifying transistor TR and the junction of the base of said PNP type output transistor TR and the shunt resistor RB in said circuit a protective resistor RS of such value that the voltage between the emitter and collector of said PNP type amplifying transistor TR may be substantially zero when the maximum allowable current flows to said PNP type output transistor TR so that said PNP type output transistor TR may be prevented from being damaged by any excess current in its abnormal operating state without directly limiting its output current.

5. A transistor output stage protecting circuit system in a so-called NPN type Darlington-connected circuit of a fundamental formation in which a load and an electric source are connected in series between the collector and emitter of a PNP type output transistor TR the emitter of an NPN type amplifying transistor TR is connected to the collector of said PNP type output transistor TR the collector of said NPN type amplifying transistor TR is connected to the base of said PNP type output transistor TR a shunt resistor RB is connected between the base and emitter of said PNP type output transistor TR; and between the base and emitter of said NPN type amplifying transistor TR is made an input terminal, the improvement which comprises inserting between the emitter of said NPN type amplifying transistor TR and the collector of said PNP type output transistor TR in said circuit a protective resistor RS of such value that the voltage between the emitter and collector of said NPN type amplifying transistor TR may be substantially zero when 7 the maximum allowable current flows to said PNP type output transistor TR, so that said PNP type output transistor TR may be prevented from being damaged by any excess current in its abnormal operating state without directly limiting its output current.

6. A transistor output stage protecting circuit system in a so-called Darlington-connected complementary type output transformerless single-ended push-pull circuit of a fundamental formation in which an electric source is connected between the collector of one TR of two PNP type output transistors TR and TR and the emitter of the other transistor TR the collector of said PNP type output transistor TR to whose emitter is connected the electric source is connected to the emitter of said PNP type output transistor TR to whose collector is connected the electric source, a load is connected through a condenser between the emitter of said PNP type transistor TR, and the emitter of said PNP type transistor TR the collector of a PNP type amplifying transistor T R is connected to the collector of said PNP type output transistor TRq, the base of said PNP type output transistor TR'] is connected to the emitter of said PNP type amplifying transistor T R a shunt resistor RB is connected between the base and emitter of said PNP type output transistor TR the emitter of an NPN type amplifying transistor TR is connected to the collector of said PNP type output transistor TR the base of said PNP type output transistor TR is connected to the collector of said NPN type amplifying transistor TR a shunt resistor RB is connected between the base and emitter of said PNP type output transistor TR the base of said NPN type amplifying transistor TR is connected to the base of said PNP type amplifying tran sistor TR and between the base of said PNP type amplifying transistor TR and the emitter of said PNP type output transistor TR7 is made an input terminal, the improvement which comprises inserting between the emitter of said PNP type amplifying transistor TR and the junction of the base of said PNP type output transistor TR7 and the shunt resistor RE in said circuit a protective resistor RS of such value that the voltage between the emitter and collector of said PNP type amplifying transistor TR may be substantially zero when the maximum allowable current flows to said PNP type output transistor TR'], and inserting between the emitter of said NPN type amplifying transistor T R; and the collector of said PNP type output transistor TR a protective resistor RS of such value that the voltage between the emitter and collector of said NPN type amplifying transistor TR may be substantially zero when the maximum allowable current flows to said PNP type output transistor TR so that said two PNP type output transistors TR, and TR may be prevented from being damaged by any excess current in their abnormal operating state without directly limiting their output current.

References Cited UNITED STATES PATENTS 2,962,665 1 1/ 1960 Greatbatch 330-40 XR 3,042,875 7/1962 Higginbotham 330-l9 XR 3,046,470 7/1962 Blocher 307-88.S 3.178,648 4/1965 Tanner 330-19 FOREIGN PATENTS 882.294 11/1961 Great Britain.

OTHER REFERENCES Lin, Quasi-Complementary Transistor Amplifier, Electronics, September 1956, pages 173-175. Copy available in 330-17 and Scientific Library.

Transistorized 6 Watt Hi-Fi, Radio-Electronics, August 1957, page 108. Copy available in 330-18 and Scientific Library.

ROY LAKE, Primary Examiner. S. H. GRIMM, F. D. PARIS, Assistant Examiners. 

1. A TRANSISTOR OUTPUT STAGE PROTECTING CIRCUIT SYSTEM IN A SO-CALLED PNP TYPE DARLINGTON-CONNECTED CIRCUIT OF A FUNDAMENTAL FORMATION IN WHICH A LOAD AND AN ELECTRIC SOURCE ARE CONNECTED IN SERIES BETWEEN THE COLLECTOR AND EMITTER OF A PNP TYPE OUTPUT TRANSISTOR TR2, THE COLLECTOR OF A PNP TYPE AMPLIFYING TRANSISTOR TR1 IS CONNECTED TO THE COLLECTOR OF SAID PNP TYPE OUTPUT TRANSISTOR TR2, THE EMITTER OF SAID PNP TYPE AMPLIFYING TRANSISTOR TR1 IS CONNECTED TO THE BASE OF SAID PNP TYPE OUTPUT TRANSISTOR TR2, A SHUNT RESISTOR RB1 IS CONNECTED BETWEEN THE BASE AND EMITTER OF SAID PNP TYPE OUTPUT TRANSISTOR TR2 AND BETWEEN THE BASE OF SAID PNP TYPE AMPLIFYING TRANSISTOR TR1 AND THE EMITTER OF SAID PNP TYPE OUTPUT TRANSISTOR TR2 IS MADE AN INPUT TERMINAL, THE IMPROVEMENT WHICH COMPRISING INSERTING BETWEEN THE COLLECTOR OF SAID PNP TYPE AMPLIFYING TRANSISTOR TR1 AND THE COLLECTOR OF SAID PNP TYPE OUTPUT TRANSISTOR TR2 IN SAID CIRCUIT A PROTECTIVE RESISTOR RS1 OF SUCH VALUE THAT THE VOLTAGE BETWEEN THE EMITTER AND COLLECTOR OF SAID PNP TYPE AMPLIFYING TRANSISTOR TR1 MAY BE SUBSTANTIALLY ZERO WHEN THE MAXIMUM ALLOWABLE CURRENT FLOWS TO SAID PNP TYPE OUTPUT TRANSISTOR TR2 SO THAT SAID PNP TYPE OUTPUT TRANSISTOR TR2 MAY BE PREVENTED FROM BEING DAMAGED BY ANY EXCESS CURRENT IN ITS ABNORMAL OPERATING STATE WITHOUT DIRECTLY LIMITING ITS OUTPUT CURRENT. 